I recently wrote my first WP plugin that adds a shortcode for embedding a custom jquery image gallery into posts. It's primarily just dumps a good chunk of HTML into the post, along with javascript necessary for initialization.

However, I had to build the HTML output procedurally, as a string in PHP. This kind of tag soup always drives me nuts, and I'm used to working with MVC frameworks which provide things like helper functions and partial templates for generating HTML.

What is the general approach for plugin authors who need to manage large amounts of dynamically built HTML or JS?

Note that the variables might be passed to a function as an array and then extract()ed or you could assign them in other ways. Also note that I use the braces not because they are always required but they make the code easier to read. (Of course with functions like the_content() being materially different from get_the_content() WordPress doesn't always make this style of coding easy.)

What's more, although it may not be relevant to you is if I use heredoc names like HTML, SQL, etc. then my IDE PhpStorm does syntax injection and will give me autocomplete and syntax coloring within the heredoc.

2.) String Concatenation using an Array

The other idiom I like to use is to collect the content into an array and then implode() the array. Although I've never benchmarked this so it could be less helpful than I assume I do know that repeated string concatenation is a killer as strings get larger (if anyone knows why this approach isn't any better or if you know a better approach I'd love to hear feedback):

Templates
========= Whenever possible, we should separate PHP from HTML. Within
Wordpress, you'll see the two mixed together without apprehension.
While this is often an "easy way" of doing things, it is almost never
the "right way." Instead, we should segregate the two, thus keeping
our logic pure and our views dumb. For this purpose we have the
$this->render('my-template') method. A few examples:

// From within a method in our controller
$this->data['message'] = 'Pass this on to the template please';
$this->render('my-template');
// Meanwhile, in the /plugin/views/my-template.php file
<h2>The Separation of Logic and Views</h2>
<p><?php echo $this->data['message'];?></p>

JavaScript

First of all, separate the dynamic parts from the static parts as much as possible. Then load whatever dynamic variables you need at runtime and queue your static scripts in the header (or footer, whatever). But this way, the majority of your JS is separate from your PHP.

HTML

This is a matter of building your code cleanly in the PHP. If you have huge chunks of HTML that you're going to re-use, I'd store those as separate variables. Then piecing things together when the shortcode is called is as simple as:

echo $firstblock;
echo $shortcodecontent;
echo $lastblock;

Rather than trying to build it out procedurally, build an object (yes, PHP supports objects) that contains all of your different HTML blocks, then instruct it which ones to use and which data to pass back. This will save you huge amounts of time.